The role of lipoprotein particles is to transport water-insoluble lipids through the bloodstream to various locations in the body. Lipoprotein particles contain proteins and lipids. A lipoprotein particle includes cholesterol, triacylglycerol (triglyceride), and phospholipids. Cholesterol and triglyceride are the two major particles found in the body. Cholesterol is a steroid metabolite that is utilized in membranes of animal cells. Triglycerides are esters that are made of glycerol and three fatty acids. Phospholipids are a major portion of cell membranes. The hydrophilic components of lipoprotein particles are found on the exterior of the lipoprotein particle. Hydrophilic components include at least portions of apolipoproteins, phospholipids, and cholesterol. The hydrophobic components are found on the interior of the lipoprotein particle and include triglycerides and cholesterol esters.
Types of lipoprotein particles include high density lipoprotein particles (HDL-P), low density lipoprotein particles (LDL-P), intermediate density lipoprotein particles (IDL-P), very low density lipoprotein particles (VLDL-P), chylomicron particles (CM-P), and lipoprotein(a) particles (Lp(a)-P). Hydrolyzed VLDL-P are called intermediate density lipoproteins (IDL-P). Each varies in size, density, protein, and lipid composition.
Classes and subclasses of apolipoproteins are apolipoprotein A (Apo A-I, Apo A-II, Apo A-IV, and Apo A-V), apolipoprotein B (Apo B-48 and Apo B-100), apolipoprotein C (Apo C-I, Apo C-II, Apo C-III, and Apo C-IV), apolipoprotein D, apolipoprotein E (Apo E-2, E-3, and E-4), and apolipoprotein H.
Different lipoprotein particles have different apolipoproteins on the surface. Apolipoproteins present in HDL-P are Apo A-I, A-II, A-IV, A-V, C-I, C-II, D, E-2, E-3, and E-4. The apolipoprotein in LDL-P is Apo B-100. Apolipoproteins in IDL-P are Apo B-100, C, E-2, E-3, and E-4. Apolipoproteins in VLDL-P are Apo A-V, B-100, C-I, C-II, C-IV, E-2, E-3, and E-4. Apolipoproteins in chylomicrons are Apo A-I, A-II, A-IV, B-48, C-I, C-II, C-III, and E-2, E-3, and E-4.
A lipoprotein(a) particle (Lp(a)-P) is an LDL-like particle with apolipoprotein A bound to apolipoprotein B by a disulfide bond. Lp(a)-P is comprised of Apo B on the surface of the LDL-like particle. Higher levels of Lp(a)-P are linked to increased risk for coronary heart disease.
Separating lipoprotein particles in bodily fluids such as serum, plasma, synovial fluid, or ascitic fluid provides information on the levels of various lipoprotein particles. Various disease states are linked to levels of apolipoproteins and/or lipoprotein particles including but not limited to cardiovascular disease, Alzheimer's disease, hyperlipidemia, abetalipoproteinemia, hypothyroidism, liver disease, diabetes mellitus, and renal problems. Higher levels of apolipoprotein B and LDL particles have been associated with increased risk of cardiovascular disease. It has been disclosed that differences in the amount of cholesterol in a particle may also play a role in the risk of cardiovascular disease: Small dense LDL-P, having more cholesterol ester, appears to be correlated with a higher risk of cardiovascular disease. However, increased levels of HDL-P correlate with a decrease in risk for cardiovascular disease.
Assay of a single type of lipoprotein particle may not be sufficient to accurately determine whether an individual is at risk for a disease because determining the total amount of a lipoprotein or apolipoprotein does not indicate with which components it is associated. For example, a particular apolipoprotein bound to a particular lipoprotein may not be indicative of a risk for developing a disease whereas the same apolipoprotein bound to a different lipoprotein indicates that the individual is at risk for that disease.
Therefore, the measurement of levels of various lipoprotein particles in bodily fluids such as serum, plasma, synovial fluid, or ascites using the combination of immunological detection and lipoprotein separation simultaneously on the same matrix indicates the risk for various disease states. Accurate predictors of the risk of an individual of developing various diseases related to lipoprotein particles are needed for research, diagnostic, and therapeutic purposes.